Flutter FutureBuilder with Completer example

main.dart

import 'package:flutter/material.dart';
import 'dart:async';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: CompleterExample(),
        ),
      ),
    );
  }
}

class CompleterExample extends StatefulWidget {
  @override
  _CompleterExampleState createState() => _CompleterExampleState();
}

class _CompleterExampleState extends State<CompleterExample> {
  final listCompleter = Completer<List<int>>();

  Future<void> _initList() async {
    await Future.delayed(Duration(seconds: 1)); //simulate the delay
    final result = List.generate(10, (index) => index);
    listCompleter.complete(result);
  }

  @override
  void initState() {
    super.initState();
    _initList();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<List<int>>(
        future: listCompleter.future,
        builder: (context, snapshot) {
          if (!snapshot.hasData) {
            return Center(
              child: CircularProgressIndicator(),
            );
          } else {
            final list = snapshot.data;
            return ListView.builder(
              itemCount: list.length,
              itemBuilder: (context, index) => Text(list[index].toString()),
            );
          }
        });
  }
}